Electronically variable beamwidth antenna

ABSTRACT

Means for obtaining an approximate 2:1 change in beamwidth electronically in an antenna system comprised of a linear array of elements. Means are coupled to one of two traveling wave tubes (TWT) which feed the array for selectively serrodyning the signal fed to said one TWT, resulting in a frequency shift of the signal feeding one half of the array with respect to the signal feeding the other half. With the serrodyning means deactivated, all of the antenna elements are fed in-phase rf signals and a conventional linear array results providing a narrow beamwidth along the electrical boresight of the array. With the serrodyning means activated and since beamwidth is inversely proportional to aperture size, each half of the antenna array is fed by separate respective coherent signals whereupon two independent beams illuminating the same angular sector will be formed having a beamwidth approximately twice that of the whole non-serrodyned array.

limited Stes Fatent [1 1 Marinaceia 1 ELECTRONKCALLY VARMBLE aaAn/iwrara ANTENNA [75 Inventor: Richard E. Marinaecio, Crofton,

[73l Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Aug. 25, 1972 [21] Appl. No.: 283,6S3

[52] US. Cl. 343/854, 343/100 SA [51] lint. Ci. H011 3/26 [58] Field of Search 343/100 R, 100 SA, 343/854 [56] Relierences Cited UNITED STATES PATENTS 3,267,472 8/1966 Fink 343/854 Primary Examiner-Eli Lieberman Attorney-1 C. Henson et al.

ON/OFF SERRODYNE fl-ELEMENT LINEAR ARRAY SlG GEN 48 [57] ABSTRACT Means for obtaining an approximate 2:] change in beamwidth electronically in an antenna system comprised of a linear array of elements. Means are coupled to one of two traveling wave tubes (TWT) which feed the array for selectively serrodyning the signal fed to said one TWT, resulting in a frequency shift of the signal feeding one half of the array with respect to the signal feeding the other half. With the serrodyning means deactivated, all of the antenna elements are fed inphase rf signals and a conventional linear array results providing a narrow beamwidth along the electrical boresight of the array. With the serrodyning means activated and since beamwidth is inversely proportional to aperture size, each half of the: antenna array is fed by separate respective coherent signals whereupon two independent beams illuminating the same angular sector will be formed having a beamwidth approximately twice that of the whole non-serrodyned array.

Ml Claims, 2 Drawing Figures WIDE BEAM A NARROW BEAM l +Al aggi/w B ELECTRONICALLY VARIABLE BEAMWIDTH ANTENNA The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Air Force (Contract F33657-70-C-1213).

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to variable beamwidth antennas and more particularly to an antenna system for airborne use having electronic control. Some radar and ECM systems require narrow elevation beamwidth for low altitude flightin order to concentrate the available power in the region of interest as well as providing a relatively small amount of main beam down-tilt while for high altitude flight a broad elevation beamwidth and a greater down-tilt is desired.

2. Description of the Prior Art In microwave antenna systems it may be desired to vary the width of the radiated beam in a predetermined manner. Widening of the beamwidth can be accomplished, for example, by defocusing the antenna such as disclosed in US. Pat. No. 3,364,490 issued to P.W. Hannan. In accordance with the invention disclosed therein, a variable beamwidth antenna for use in a system which derives information depending on phase relationships existing between different portions of the received signal and comprises: an antenna, first means for changing the focusing effect provided by the antenna, and second means coupled to the first means for providing phase shift compensation related to the change in focusing effect produced by the first means.

Widening the beamwidth of an antenna can also be accomplished by reducing the effective diameter of the radiating aperture; however, such methods heretofore have been usually inefficient and complicated and subject to various defects related to operation with reduced aperture.

Additionally, beam shaping can also be accomplished by the method of sidelobe suppression, such as taught in US. Pat. No. 3,325,816 issued to FA. Dutton.

SUMMARY Briefly, the subject invention is directed to an improved variable beamwidth antenna system comprised of an n element linear array wherein n/2 linearly arranged elements on one side of the array are fed from a first rf amplifier (TWT) and the other n/2 linearly arranged elements on the other side of the array are fed from a second rf amplifier (TWT). A common rf signal source is coupled to both rf amplifiers and in a first mode of operation, all n elements are fed in-phase to provide a beam of relatively narrow beamwidth along the array boresight. Means are coupled to one of the rf amplifiers in a second mode of operation for serrodyning the rf signal applied to said one rf amplifier causing a frequency shift of the rf signal fed to the n/2 elements fed from the serrodyned amplifier whereupon each n/2 elements operate as a first and second linear sub-array having a respective reduced aperture and thereby providing two independent beams illuminating the same angular sector having a beamwidth being approximately twice that of the n elements operated in the first mode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of the preferred embodiment of the present invention; and

FIG. 2 is a diagram illustrative of the narrow and broad radiation pattern provided by the subject invention operated in a first and second mode, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the schematic diagram shown in FIG. 1, reference numeral 10 represents an airborne linear antenna array comprised of n microwave radiating elements arranged in equally spaced relationship and oriented to provide a radiation pattern, for example, in elevation when suitable rf microwave signals are applied thereto. In the configuration shown in FIG. l, n 4. One half (n/2) of the elements in the four element array are designated by reference numeral 112 and 12 while the other half of the array is comprised of the elements I4 and 14 Radiating elements 12 and 12 are coupled to an rf power coupler 16 referred to as a power splitter by means of microwave feeder elements 18 and 20, respectively which are comprised of, for example, waveguides. An rf amplifier 22 of microwave signals, such as a traveling wave tube (TWT), is coupled to the power splitter 16 by means of the feeder element 24. TWT amplifier 22 has its input coupled back to a primary power splitter 26 by means of feeder element 28. The power splitter 26 is coupled to an initial TWT amplifier by means of feeder element 30. The input of TWT amplifier 28 is coupled to suitable rf input means 32 by means of feeder element 34.

The other half of the array which comprise elements 14 and 14 are coupled to the primary power splitter 26 in a manner similar to the elements 12 and I2 described above. More particularly, radiating elements 14 and 14 are coupled by means of respective feeder elements 36 and 38 to a power splitter 40 which has its input connected to the output of TWT amplifier 42 by means of feeder element 44. The input to TWT amplifier 42 is coupled back to the power splitter 26 by means of feeder element 46. TWT amplifier 42 additional ly is adapted to be controlled by means of a serrodyne signal generator 48 by means of the electrical signal circuit connection 50. The serrodyne signal generator 48, moreover, is adapted to be coupled toa control terminal 52 whic is adapted to receive either an ON or OFF control signal which causes the signal generator 48 to respectively generate or not generate" a repetitive sawtooth waveform which is coupled to the circuit lead 50 for modulating the TWT amplifier 42. When an ON signal is applied to terminal 52, a sawtooth waveform will be produced by the signal generator 48 which is coupled by means of circuit lead 50 to the TWT-42 for serrodyning the :rf energy fed to TWT 42 from the power splitter 26. The rf energy in the TWT 42 is serrodyne modulated by the sawtooth output from serrodyne signal generator 48. Serrodyne modulation results in a continuous linear phase shift repeated every 211' radians. Such a phase shift causes the energy in the original carrier frequency to be displaced or shifted by a predetermined constant amount equal to the frequency of the modulating signal, that is, the frequency of the sawtooth waveform appearing on circuit lead 50 when an ON signal is applied to terminal 52. When an OFF signal appears at terminal 52, signal generator 48 is deactivated and the serrodyne modulating signal is removed from TWT 42. The concept of serrodyne modulation is further explained in US. Pat. No. 3,058,049 entitled Serrodyne Frequency Shifters issued to F. J. OHara, et al.

Bearing the foregoing in mind, in the normal or first mode of operation, the serrodyne modulating sawtooth waveform is absent due to the application of an OFF control signal to terminal 52 coupled to serrodyne signal generator 48. The rf input signal to be radiated is applied to input means 32 where it is fed through TWT amplifier 28 to the primary power splitter 26. The rf signal is then fed into two separate channels which include TWT amplifiers 22 and 42, respectively. The rf ouput appearing in feeder arms 24 and 44 are in-phase whereupon it is respectively fed to power splitters 16 and 40. Antenna elements 12 and 12 are fed in-phase rf signals from the power splitter 16 while antenna elements 14 and 14 are fed the same in-phase rf signals from power splitter 40. Thus a linear array comprised of n 4 elements radiate a relatively narrow beam alongthe boresight axis having a virtual source designated by reference numeral 52 as shown in FIG. 1. Since the beamwidth is inversely proportional to the diameter of the radiating aperture wherein the aperture of the array comprises the number of elements times the aperture for a single element, the beam is narrower than if a smaller number than n elements comprise the array.

The second mode of operation is directed to radiatin'ga relatively wider beamwidth and to this end an ON control signal is now applied to terminal 52 whereupon the signal generator 48 couples a repetitive sawtooth waveform to the TWT 42. The rf signal to be radiated is again fed from the primary power splitter 26 and from there to the TWT amplifier 22 and to the TWT amplifier 42. TWT 42 is now serrodyned modulated by the sawtooth waveform applied thereto from the signal generator 48. As noted above, serrodyne modulation causes a frequency shift of the rf signal. Therefore, the rf signal to be radiated from antenna elements 14 and 14 is shifted in frequency from that radiated from antenna elements 12 and 12 This causes the array to be effectively operated as two separate antenna sections, or sub-arrays, one of which includes elements 12 and 12 while the other comprises antenna elements 14 and 14 The rf signal applied to power splitter 16 is applied to the elements 12 and 12 in phase and thus effectively becomes a linear n/2 radiator sub-array having a virtual source 54 as shown in FIG. 1 and which exhibits a relatively wide beamwidth A due to the fact that the'number of antenna elements is reduced by onehalf. in other words, the aperture size is reduced by one-half resulting in a relatively wider beam radiated from elements 12 and 12 On the other hand, the rf shifted in frequency is applied to power splitter 40 and a different in-phase rf is fed to elements 14, and 14 providing a virtual source 56 comprised of n/2 elements which exhibits a second relatively wide beamwidth B. The resulting effect is the formation of two separate beams A and B illuminating the same angular sector, beam A from the upper sub-array and one from the lower sub-array. At operational distances, however, wide beams A and B overlap and appear to emanate from the same virtual source 52 as the narrow beam as shown in FIG. 2. Thus control of the beamwidth is obtained by turning the serrodyne modulated signal on and off, that is ON for the broadbeam and OFF for the narrow beam.

The subject invention has particular application for airborne ECM systems where the TWT amplifiers 22 and 42 receive signal common sinal which can be either a noise signal or a received radar signal which is to be reradiated (repeater operation). The line lengths and TWT phase shifts are normally matched so that all antenna elements will be fed in-phase in the narrowband mode and a conventional linear array will result providing a narrow beamwidth with the electrical boresight of the array corresponding to the mechanical boresight of the array. The wider beamwidth is provided in the second mode of operation by serrodyning TWT amplifier 42, resulting in a frequency shift of the signal feeding one half of the array with respect to the signal feeding the other half. Since in the second mode of operation two beams are formed, the resulting composite pattern will have the average power equal to the sum of that from each beam. The composite signal will also be amplitude modulated at a rate corresponding to the serrodyne rate, however, if the rate is high compared to the intended jamming modulation, no significant degradation will result. The net result is therefore an in-flight electronic beamwidth control.

Having disclosed what is at present considered to be the preferred embodiment of the subject invention,

I claim:

1. A variable beamwidth antenna fed from a source of rf signals, comprising in combination:

an antenna array of n antenna elements arranged and adapted to be operated in one mode of operation as a first and second adjacent n/2 element subarray and in another mode of operation as a single n element array;

input means coupled from said source of rf signals;

a first feeder circuit coupling said input means to said first n/2 element subarray;

a second feeder circuit coupling said input means to said second n/2 element subarray; and

serrodyne circuit means coupled to said second feeder circuit, being operable in said one mode of operation to serrodyne modulate rf signals fed to said second feeder circuit for causing an rf signal to be radiated from said second n/2 element subarray in a relatively wide beamwidth radiation pattern and shifted in carrier frequency with respect to the rf signal radiated from said first n/2 element subarray also having a relatively wide beamwidth radiation pattern, said serrodyne circuit means being inoperative in said another mode of operation wherein said first and second feeder circuits feed common in-phase rf signals from said source to the respective n/2 element subarray thereby effecting a composite n element antenna array radiating a relatively narrow beam of a single carrier frequency.

2. The antenna as defined by claim 1 wherein said antenna array comprisesa linear microwave array.

3. The antenna as defined by claim 2 wherein said second feeder circuit includes an RF amplifier.

4. The antenna as defined by claim 3 wherein said RF amplifier comprises a traveling wave tube.

5. The antenna as defined by claim 4 and additionally including a traveling wave tube in said first feeder circuit.

6. The antenna as defined by claim 1 wherein said serrodyne circuit means includes a sawtooth waveform signal generator selectively rendered operative in said one mode of operation, and inoperative in said another mode of operation.

7. The antenna as defined by claim ll wherein said input means includes a first RF power splitter coupled from said source of rf signals to said first and second feeder circuit, and

additionally including a second and third power splitter respectively coupling said first and second feeder circuit to said first and second n/2 element sub-array.

8. The antenna as defined by claim 7 wherein said input means additionally includes an rf amplifier coupling said source of rf signals to said first rf power splitter.

9. The antenna as defined by claim 8 wherein said antenna array comprises a linear microwave array and said rf amplifier comprises a traveling wave tube, and

additionally including a traveling wave tube respectively in said first and second feeder circuit.

10. The antenna as defined by claim 1 wherein said first and second feeder circuit include feeder elements having line lengths which are substantially equal for providing rf signals which are in phase at said n elements during said another mode of operation. 

1. A variable beamwidth antenna fed from a source of rf signals, comprising in combination: an antenna array of n antenna elements arranged and adapted to be operated in one mode of operation as a first and second adjacent n/2 element subarray and in another mode of operation as a single n element array; input means coupled from said source of rf signals; a first feeder circuit coupling said input means to said first n/2 element subarray; a second feeder circuit coupling said input means to said second n/2 element subarray; and serrodyne circuit means coupled to said second feeder circuit, being operable in said one mode of operation to serrodyne modulate rf signals fed to said second feeder circuit for causing an rf signal to be radiated from said second n/2 element subarray in a relatively wide beamwidth radiation pattern and shifted in carrier frequency with respect to the rf signal radiated from said first n/2 element subarray also having a relatively wide beamwidth radiation pattern, said serrodyne circuit means being inoperative in said another mode of operation wherein said first and second feeder circuits feed common in-phase rf signals from said source to the respective n/2 element subarray thereby effecting a composite n element antenna array radiating a relatively narrow beam of a single carrier frequency.
 2. The antenna as defined by claim 1 wherein said antenna array comprises a linear microwave array.
 3. The antenna as defined by claim 2 wherein said second feeder circuit includes an RF amplifier.
 4. The antenna as defined by claim 3 wherein said RF amplifier comprises a traveling wave tube.
 5. The antenna as defined by claim 4 and additionally including a traveling wave tube in said first feeder circuit.
 6. The antenna as defined by claim 1 wherein said serrodyne circuit means includes a sawtooth waveform signal generator selectively rendered operative in said one mode of operation, and inoperative in said another mode of operation.
 7. The antenna as defined by claim 1 wherein said input means includes a first RF power splitter coupled from said source of rf signals to said first and second feeder circuit, and additionally including a second and third power splitter respectively coupling said first and second feeder circuit to said first and second n/2 element sub-array.
 8. The antenna as defined by claim 7 wherein said input means additionally includes an rf amplifier coupling said source of rf signals to said first rf power splitter.
 9. The antenna as defined by claim 8 wherein said antenna array comprises a linear microwave array and said rf amplifier comprises a traveling wave tube, and additionally including a traveling wave tube respectively in said first and secoNd feeder circuit.
 10. The antenna as defined by claim 1 wherein said first and second feeder circuit include feeder elements having line lengths which are substantially equal for providing rf signals which are in phase at said n elements during said another mode of operation. 